Pseudorabies virus (PRV) is a disease which infects many species of animals worldwide. PRV infections are variously called infectious Bulbar paralysis, Aujesky's disease, and mad itch. Infections are known in important domestic animals such as swine, cattle, dogs, cats, sheep, rats and mink. The host range is very broad and includes most mammals and, experimentally at least, many kinds of birds (for a detailed list of hosts, see D. P. Gustafson, "Pseudorabies", in Diseases of Swine, 5th ed., A. D. Leman et al., eds., (1981)). For most infected animals the disease is fatal. Adult swine and possibly rats, however, are not killed by the disease and are therefore carriers for the disease.
Populations of swine are particularly susceptible to PRV. Although the adult swine rarely show symptoms or die from the disease, piglets become acutely ill when infected and death usually ensues in 24 to 48 hours often without specific clinical signs (T. C. Jones and R. D. Hunt, Veterinary Pathology, 5th ed., Lea & Febiger (1983)).
PRV vaccines have been produced by a variety of techniques and vaccination in endemic areas of Europe has been practiced for more than 15 years. Losses have been reduced by vaccination, but vaccination has maintained the virus in the environment. No vaccine has been produced that will prevent infection. Vaccinated animals that are exposed to virulent virus survive the infection and then shed more virulent virus. Vaccinated animals may therefore harbor a latent infection that can flare up again. (See, D. P. Gustafson, supra).
Live attenuated and inactivated vaccines for PRV are available commercially in the United States and have been approved by the USDA (see, C. E. Aronson, ed., Veterinary Pharmaceuticals & Biologicals, (1983)).
Because adult swine are carriers of PRV, many states have instituted screening programs to detect infected animals. A problem arises in distinguishing between those animals carrying virulent PRV and those which have been vaccinated. The antigenic profiles of the virulent viruses and the viruses used in vaccines are the same and therefore it may be impossible to distinguish between infected and vaccinated animals. As a result, regulations concerning movement of seropositive swine would apply to both vaccinated swine and to swine that have been previously infected with PRV (C. E. Aronson, supra.).
PRV is a herpesvirus. The herpesviruses generally are among the most complex of animal viruses. Their genomes encode at least 50 virus specific proteins and contain upwards of 150,000 nucleotides. Among the most immunologically reactive proteins of herpesviruses are the glycoproteins found, among other places, in virion membranes and the membranes of infected cells. The literature on PRV glycoproteins refers to at least four viral glycoproteins (T. Ben-Porat and A. S. Kaplan, Virology, 41, pp 265-73 (1970); A. S. Kaplan and T. Ben-Porat, Proc. Natl. Acad. Sci. USA, 66, pp. 799-806 (1970)).
Several herpesviruses reportedly secrete glycoproteins into the medium of infected cells. Herpes simplex virus (HSV) releases glycoprotein C and several truncated forms of glycoprotein D into the medium (B. Norrild and B. F. Vestergaard, Intervirology, 11, pp. 104-10 (1979); R. E. Randall, et al., J. Gen. Virol., 48, pp. 297-310 (1980)). Marek's disease virus releases a considerable amount of the virion glycoprotein A into the medium (D. Van Zaane, et al., Virology, 121, pp. 116-32 (1982)); and herpes saimiri virus also releases a virion glycoprotein in the medium (R. E. Randall and R. W. Honess, J. Gen. Virol., 51, pp. 445-49 (1980)). PRV releases a glycoprotein into the medium which reportedly is not incorporated into the viral particles (T. Ben-Porat and A. S. Kaplan, Virology, 41, pp. 265-73 (1970); T. J. Rea, et al., J. Virol., 54, pp. 21-29 (1985)).
The PRV protein which is secreted into the medium has been referred to as 3a (T. Ben-Porat and A. S. Kaplan, supra), and is also referred to as glycoprotein X (gX) (T. J. Rea, et al., supra.). gX has the following characteristics when isolated from PRV-infected cells:
(1) it is the predominant protein in the culture medium of PRV infected animal cells in culture;
(2) it is a glycoprotein;
(3) it has a molecular weight of about 95 kd on SDS polyacrylamide gels;
(4) it is a sulfated protein;
(5) it is soluble in about 1% perchloric acid; and
(6) it is immunogenic in standard laboratory mice.
The instant invention overcomes the problems referred to above, for example in screening swine for PRV infection, by providing a PRV strain which is immunologically distinct from the wild-type virus, thus allowing one to distinguish between vaccinated and infected animals without the need for sacrificing the tested animals.
These antigenic differences may be a result of deletion of one or more detectable antigenic polypeptides from the vaccine virus. As a result of these genetic changes, it is possible to immunologically distinguish between infected and vaccinated animals on the basis of their serological profiles without the need for sacrificing the tested animals.